Updates comment for SetAction

mailcap.go

@@ -123,6 +123,12 @@ func (e Entry) Execute(path string) error {
 	return nil
 }
 
+// Sets the action (key) for the entry calling it. This action
+// is used when calling execute. The action is generally set by the
+// Mailcap receiver FindMatch and SetAction is provided in order
+// to easily manipulate an Entry found in the db directly. This
+// is not recommended, but is provided as a lower level way of 
+// using the lib.
 func (e Entry) SetAction(action string) error {
 	if _, ok := e[action]; !ok {
 		return fmt.Errorf("This entry does not have the action %q available", action)